It has been known that an electromagnetic shielding film (sometimes simply referred to as “shielding film”) is arranged on the surface of a flexible printed wiring board for shielding an electromagnetic noise generated from the printed wiring board or an electromagnetic noise from the outside.
In recent years, along with an increase in transmission speed of a portable electronic device, a material to be used in a printed wiring board has started to be required to have excellent high-speed transmission characteristics, more specifically, characteristics such as a low specific dielectric constant and a low dielectric loss tangent. As a result, the shielding film has also been required to have such excellent high-speed transmission characteristics. Meanwhile, the shielding film is often required to have flame retardancy. The use of a non-halogen-based flame retardant in a conductive adhesive to be used in the shielding film has been known as a technology for the impartment of flame retardancy to the shielding film (Patent Literature 1). However, the shielding film having imparted thereto flame retardancy involves a problem in that its specific dielectric constant and dielectric loss tangent are not enough to satisfy levels required by the increase in transmission speed.
As described above, a shielding film that simultaneously satisfies excellent high-speed transmission characteristics and excellent flame retardancy has been strongly required.